Ta có: \(\widehat{A}\) + \(\widehat{B}\) +\(\widehat{C}\) = 180 0( tổng 3 góc của ΔABC )
=>\(\widehat{B}\) + \(\widehat{C}\) = 100 ( Do \(\widehat{A}\) = 800 )
=> \(\dfrac{1}{2}\) (\(\widehat{A}\) + \(\widehat{B}\) ) = \(\dfrac{1}{2}\)100
=>\(\dfrac{1}{2}\)\(\widehat{B}\) + \(\dfrac{1}{2}\)\(\widehat{C}\) = 800
Xét Δ BIC ta có:
\(\widehat{BIC}\) + \(\widehat{B}\)1 + \(\widehat{C}\)1 = 1800
=>\(\widehat{BIC}\) + 800 = 180 0
=>\(\widehat{BIC}\) = 1800 - 800
=>\(\widehat{BIC}\) = 1000
Vậy : \(\widehat{BIC}\) = 1000
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